http://www.ck12.org Chapter 2. Matter
Same Element, Different Numbers of Neutrons
All the atoms of a given element have the same number of protons and electrons. The number of neutrons, however,
may vary for atoms of the same element. For example, almost 99 percent of carbon atoms have six neutrons, but
the rest have either seven or eight neutrons. Atoms of an element that differ in their numbers of neutrons are called
isotopes. The nuclei of these isotopes of carbon are shown in theFigure2.57. The isotope called carbon-14 is used
to find the ages of fossils. You can learn how at this URL: http://science.howstuffworks.com/environmental/earth
/geology/carbon-14.htm.
FIGURE 2.57
Q:Notice the names of the carbon isotopes in the diagram. Based on this example, infer how isotopes of an element
are named.
A:Isotopes of an element are named for their total number of protons and neutrons.
Q:The element oxygen has 8 protons. How many protons and neutrons are there in oxygen-17?
A:Oxygen-17—like all atoms of oxygen—has 8 protons. Its name provides the clue that it has a total of 17 protons
and neutrons. Therefore, it must have 9 neutrons (8 + 9 = 17).
Particles in Neutrons
Neutrons consist of fundamental particles known as quarks and gluons. Each neutron contains three quarks, as
shown in the diagram below. Two of the quarks are called down quarks (d) and the third quark is called an up quark
(u). Gluons (represented by wavy black lines in the diagram) are fundamental particles that are given off or absorbed
by quarks. They carry the strong nuclear force that holds together quarks in a neutron.
Summary
- A neutron is one of three main particles that make up the atom. It is found in the nucleus and is neutral in
electric charge. It has about the same mass and diameter as a proton. Neutrons are found in all atoms except
for most atoms of hydrogen. - All the atoms of a given element have the same number of protons and electrons, but they may vary in their
numbers of neutrons. Atoms of the same element that differ in their numbers of neutrons are called isotopes. - Neutrons consist of fundamental particles known as quarks and gluons. Gluons carry the strong nuclear force
that binds together the quarks in a neutron.